In semiconductor industries, strong efforts are made to bring new promising memory technologies based on non-volatile MRAM cells into practical (commercial) use. An MRAM cell includes a stacked structure of magnetic layers separated by a non-magnetic tunneling barrier layer and arranged into a magnetic tunnel junction (MTJ). Digital information is not maintained by power as in conventional DRAMs, but rather by specific directions of magnetizations (magnetic moments or magnetic moment vectors) in the ferromagnetic layers. More specifically, in an MRAM cell, magnetization of one ferromagnetic layer (called “reference layer” or “pinned layer”) is magnetically fixed or pinned, and, magnetization of the other ferromagnetic layer (called “free layer”) is free to be switched between two preferred directions along an easy axis (preferred axis) of magnetization thereof, which directions typically are in a same or opposite alignment as to the reference layer's fixed magnetization.
Depending upon the magnetic states of the free layer (i.e., parallel or antiparallel alignment of its magnetization with respect to the magnetization of the reference layer), the magnetic memory cell exhibits two different resistance values in response to a voltage applied across the magnetic tunnel junction barrier, where the resistance thereof is “low” when magnetizations are in parallel alignment and “high” when magnetizations are in antiparallel alignment. Accordingly, logic information corresponding to one logic bit may be assigned to the different magnetizations of the free layer, and a simple detection of electric resistance provides logic information stored in the magnetic memory element.
An MRAM cell is written to through the application of magnetic fields created by bi- or unidirectional currents made to run through current lines arranged adjacent the MRAM cell so that their magnetic fields are coupled to the free layer magnetization. More specifically, if a magnetic field in a direction opposite to the magnetization of the free layer is applied, then magnetization of the free layer is reversed in case a critical magnetic field value is reached, which is also referred to as reversal magnetic field. The value of the reversal magnetic field can be determined from a minimum energy condition. Assuming that a magnetic field applied to the direction of the hard axis of magnetization (that is in orthogonal alignment to the easy axis thereof) is represented by Hx and a magnetic field applied to the easy axis of magnetization is represented by Hy, then a relationship Hx(2/3)+Hy(2/3)=Hc(2/3) is established, where Hc represents the anisotropic magnetic field of the free layer. Since this curve forms an astroid on a Hx-Hy-plane, it is called an astroid curve. Hence, a composite magnetic field enables the selection of a single MRAM-cell in case the sum of both magnetic fields at least amounts to the reversal magnetic field. Based upon such condition, a single MRAM cell can be switched using combined magnetic fields (which is known, for example, as a “Stoner-Wohlfahrt” switching scenario).
Recently, a new concept of MRAM cells has been proposed, in which a ferromagnetic free region includes a plurality of ferromagnetic free layers that are antiferromagnetically coupled, where the number of antiferromagnetically coupled free layers may be appropriately chosen to increase the effective magnetic switching volume of the MRAM device. See, for example, U.S. Pat. No. 6,531,723, European Patent No. 674769 and German Patent Application No. 4243358, the disclosures of which are incorporated herein by reference in their entireties. For switching such magnetoresistive memory cells, another switching scenario, the so-called “adiabatic rotational switching” may be used. An example of this switching technique is described in U.S. Pat. No. 6,545,906, the disclosure of which is incorporated herein by reference in its entirety.
In short, adiabatic rotational switching relies on a “spin-flop” phenomenon, which lowers the total magnetic energy in an applied magnetic field by rotating the magnetic moment vectors of the antiferromagnetically coupled ferromagnetic free layers. More specifically, assuming that a first magnetic field HBL of a first current line (e.g. bit line) and a second magnetic field HWL of a second current line (e.g. word line) respectively arrive at an MRAM cell for the switching thereof, and that antiferromagnetically coupled magnetizations M1 and M2 of the free layers are inclined at a 45° angle to the word and bit lines, respectively, a timed switching pulse sequence of magnetic fields to be applied in a typical “toggling write” mode is as follows. At a time t0 neither a word line current nor a bit line current are applied resulting in a zero magnetic field H0 of both HBL and HWL. At a time t1, the word line current is increased to H1 and magnetic moment vectors M1 and M2 begin to rotate either clockwise or counter-clockwise, depending on the direction of the word line current. At a time t2, the bit line current is switched on, where it is chosen to flow in a certain direction so that both magnetic moment vectors M1 and M2 are further rotated in the same clockwise or counter-clockwise direction as the rotation caused by the word line magnetic field. At this time t2, both the word and bit line currents are on, resulting in magnetic field H2 with magnetic moment vectors M1 and M2 being nominally orthogonal to the net magnetic field direction, which is 45° with respect to the current lines. At a time t3, the word line current is switched off, resulting in magnetic field H3, so that magnetic moment vectors M1 and M2 are being rotated only by the bit line magnetic field. At this point in time, magnetic moment vectors M1 and M2 have generally been rotated past their hard axis instability points. Finally, at a time t4, the bit line current is switched off, again resulting in zero magnetic field H0, and magnetic moment vectors M1 and M2 will align along the preferred anisotropy axis (easy axis) in a 180° angle rotated state as compared to the initial state. Accordingly, with regard to the magnetic moment vector of the reference layer, the MRAM cell has been switched from its parallel state into its anti-parallel state, or vice versa, depending on the initial switching (“toggling”) state.
In order to successfully switch an MRAM cell, in a coordinate plane spanned by HWL and HBL, it is a first precondition that a magnetic field sequence applied thereon results in a magnetic field path crossing a diagonal line spanned between a first critical value (called “spin-flop magnetic field” HSF or “toggling point (T)”) for initiating toggle switching and a second critical point (called “saturation magnetic field” HSAT) where antiferromagnetic coupling between ferromagnetic free layers is nullified. It is further a second precondition that the magnetic field sequence circles around the spin-flop magnetic field, since only in this case magnetic moment vectors M1 and M2 are rotated past their hard axis instability points.
In view of modem portable equipment, such as portable computers, digital still cameras and the like, requiring very large memory performance, one of the most important issues for MRAM cells is a down-sizing thereof. However, in the case of antiferromagnetically coupled free layers, such down-sizing results in a dramatic increase of antiferromagnetic coupling forces therebetween.
Referring to FIG. 1, a patterned layered stack of a conventional MRAM memory element is depicted including antiferromagnetically coupled ferromagnetic free layers. In such structure, on a metallic base material 1 which typically is connected to an active structure of a semiconductor substrate (not shown), a ferromagnetic reference region 2, a tunneling barrier 3 made of a non-magnetic material, and a ferrommagnetic free region 4 including ferromagnetic free layer 5 and ferromagnetic free layer 6 separated by a relatively thick spacer layer 7 are provided. The reference region 2 has a fixed (pinned) magnetization adjacent the tunneling barrier layer 3, while the free region 4 includes ferromagnetic free layers 5, 6 having free magnetizations 8, 9 that are antiferromagnetically coupled and can be switched in parallel or anti-parallel alignment as to the fixed magnetization. Optionally, an underlayer 10 is arranged below the reference layer region 2 and a a cap layer 11 is arranged above the ferromagnetic free layer region 4.
The ferromagnetic free region 4 which includes ferromagnetic free layers 5, 6 and spacer layer 7 are assumed to have height r. As can be shown by numeric simulations which need not be further detailed here, a relationship between a varied thickness of spacer layer 7 (resulting in a change of magnetic free region height r) and spin-flop magnetic field as well as saturation magnetic field for toggle switching is established. Accordingly, decreasing a spacer layer 7 thickness (which is a decrease of height r) results in an increase of both spin-flop and saturation magnetic fields due to an increase of magnetic dipole coupling energy being proportional to 1n(r). Hence, in order to avoid high switching currents for toggling the ferromagnetic free layers, a thick spacer layer 7 is preferred. However, as noted above, this is detrimental to down-scaling of the memory cells.